FIG. 1 shows a typical wireless LAN/MAN system with a radio frequency (RF) unit 104 coupled to baseband processor 118. The receive path includes RF front end 106 which amplifies received signals, which are then analog filtered 110 and digitized 112 at a sample rate Fs. The incoming samples at rate Fs are decimated by filter 120 to generate a decimator output 124 at rate Fs/2. Transmit data 126 at rate Fs/2 is provided to interpolation filter 122 which increases the incoming sample rate from Fs/2 126 to Fs, and those samples are applied to DAC 116, filtered 114, and applied to Tx Front End 108, which amplifies, upconverts, and couples to antenna 102. Typically the ADC and DAC sampling rate (Fs) is an integer multiple of the sample rate of the baseband processor generating transmit data 126 or receiving data 124, which simplifies the suppression of images caused by ADC 112 and DAC 116 sampling. The analog filter 110 and 114 requirements are also relaxed as these filters can be designed with a larger transition band. The residual image is filtered digitally in the baseband by the interpolation 122 and decimation 120 filters.
In the receive path, the decimation filter 120 removes the residual image and then down samples the input signal to the baseband sampling rate. In the transmit path, the interpolator 122 up-samples the baseband signal by inserting zeros in alternate samples, which are then filtered in the upsampled signal to remove images.
Interpolation filter 122 and decimation filter 120 are typically implemented in finite impulse response (FIR) filters, rather than infinite impulse response (IIR) filters in OFDM baseband processors. IIR filters have a register configuration where computed terms are fed back to earlier registers, which results in greater hardware efficiency and the need for fewer storage registers than FIR filters for the same spectrum shaping requirements. The drawback of IIR filters is the introduction of inter-8 symbol interference (ISI), hence degrading the performance of the wireless OFDM link in the presence of multipath reflection. FIG. 2A shows the affect of the increased filter impulse response on inter-symbol interference. A signal 202 represents the input signal at a first tap point of a multipath IIR filter, and signal 204 represents the input signal at a subsequent filter tap point. The FIR filter impulse response is shown in waveform 212, and the corresponding filter tail time response 214 is shown at the same time resolution as incoming data, which includes symbol S1206 followed by cyclic prefix 208 and symbol S2 210. As can be seen from S1 region 214 representing the part of S1 which undesirably contributes to S2 filter output, a small part of the FIR response 212 from symbol S1 is bleeding symbol S2. The filter impulse response adds to the delay spread caused by multipath and the overall spread can exceed the cyclic prefix 208. The part of the resulting delay spread that exceeds the cyclic prefix contributes to ISI. FIG. 2B shows the much larger ISI effect of IIR filters, where the impulse response 244 includes the much larger extent of S1 associated with the IIR filter tail extent 242 extending well into S1 which adds to the S2 response. The filter tail 242 which extends into S1 for an IIR filter as shown in FIG. 2B results in much greater ISI of S1 into S2 than the FIR filter of FIG. 2A.
It is desired to utilize a filter for receive decimation and for transmit interpolation, where the filter has a smaller number of taps such as an IIR filter, but without the excessive time response and related ISI associated with an IIR filter.